Interdisciplinary studies for the knowledge of the groundwater fluoride contamination in the eastern African rift: Meru district - North Tanzania

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(1)UNIVERSITY OF SASSARI DISSERTATION OF THE DEGREE OF PhD SCHOOL IN NATURAL SCIENCE UNIVERSITY OF SASSARI, 2010 XXII CYCLE. INTERDISCIPLINARY STUDIES FOR THE KNOWLEDGE OF THE GROUNDWATER FLUORIDE CONTAMINATION IN THE EASTERN AFRICAN RIFT: MERU DISTRICT – NORTH TANZANIA. Candidate: Daniele Pittalis Supervisor: Prof. Marco Apollonio. Tutor:. Prof. Giorgio Ghiglieri. Co-Tutor:. Prof. Giacomo Oggiano.

(2) Daniele Pittalis – Interdisciplinary studies for the knowledge of the groundwater fluoride contamination in the eastern African rift: Meru district, North Tanzania – Natural Science PhD School, University of Sassari. 2.

(3) Dedicated to my love:. Angela For a lot of beautiful sensations and mainly because… I love you thank you, Ange. Daniele Pittalis – Interdisciplinary studies for the knowledge of the groundwater fluoride contamination in the eastern African rift: Meru district, North Tanzania – Natural Science PhD School, University of Sassari. 3.

(4) Daniele Pittalis – Interdisciplinary studies for the knowledge of the groundwater fluoride contamination in the eastern African rift: Meru district, North Tanzania – Natural Science PhD School, University of Sassari. 4.

(5) Groundwater studies do not create more water, but good information can prevent costly mistakes and can help increase the efficiency of resource use. Sustainable Development of groundwater resources in southern and eastern Africa- Regional Technical cooperation project RAF/8/029. Daniele Pittalis – Interdisciplinary studies for the knowledge of the groundwater fluoride contamination in the eastern African rift: Meru district, North Tanzania – Natural Science PhD School, University of Sassari. 5.

(6) LIST OF CONTENENTS. 1.. INTRODUCTION AND OBJECTIVES ............................................................... 7. 2.. LITERATURE REVIEW ................................................................................. 13 2.1. FLUORIDE CONTAMINATION........................................................................... 13 2.1.1. Air ................................................................................................... 13 2.1.2. Soil .................................................................................................. 14 2.1.3. Water sea ......................................................................................... 14 2.1.4. Surface water .................................................................................... 14 2.1.5. Groundwater and its hydrogeochemistry ............................................... 14 2.2. FLUORIDE HEALTH PROBLEMS ........................................................................ 18 2.3. FLUORIDE IN THE RIFT VALLEY AND IN TANZANIA ................................................. 21. 3.. DESCRIPTION OF THE STUDY AREA ........................................................... 23 3.1. LOCATION AND EXTENT ............................................................................... 23 3.1.1. The Rift Valley ................................................................................... 24 3.2. HYDROMETEOROLOGY ................................................................................. 30 3.3. GEOMORPHOLOGY ..................................................................................... 31 3.4. GEOLOGY ............................................................................................... 32 3.4.1. Litostratigraphy ................................................................................. 32 3.4.2. Geological Structure ........................................................................... 35 3.5. HYDROGEOLOGY ....................................................................................... 36. 4.. MATERIALS AND METHODS ........................................................................ 39 4.1. FIELD DATA COLLECTION .............................................................................. 39 4.1.1. Census of water points ....................................................................... 39 4.1.2. Masika and pre-masika monitoring activity ............................................ 40 4.1.3. Geophysical surveys ........................................................................... 43 4.2. LABORATORY ANALYSIS ............................................................................... 43 4.3. INVERSE GEOCHEMICAL MODELING .................................................................. 44 4.4. CHEMICAL AND MINERALOGICAL DATA .............................................................. 45. 5.. INFERENCE FROM THE ANALYTICAL RESULTS ............................................ 58 5.1. GEOPHYSICAL DATA ................................................................................... 58 5.2. VOLCANIC ROCKS AND DERIVED SEDIMENTS CLASSIFICATION .................................... 60 5.2.1. Rocks ............................................................................................... 60 5.2.2. Sediments ........................................................................................ 63 5.3. ROCK MINERALOGY .................................................................................... 64 5.4. GROUNDWATER AND SURFACE WATER RESULTS .................................................... 65 5.4.1. Masika monitoring .............................................................................. 65 5.4.2. Pre-Masika monitoring ........................................................................ 69 5.5. ISOTOPIC DATA ........................................................................................ 78 5.5.1. River and Lake water samples.............................................................. 79 5.5.2. Groundwater samples ......................................................................... 79 5.6. INVERSE GEOCHEMICAL MODELING .................................................................. 81. 6.. DISCUSSION .............................................................................................. 87. 7.. CONCLUSION ............................................................................................. 90. REFERENCES ..................................................................................................... 93. Daniele Pittalis – Interdisciplinary studies for the knowledge of the groundwater fluoride contamination in the eastern African rift: Meru district, North Tanzania – Natural Science PhD School, University of Sassari. 6.

(7) 1. INTRODUCTION AND OBJECTIVES Water contamination is a global problem that affect development and sub-development countries. Particularly groundwater pollution, as a result of human activities and natural contamination, has become one of the most debated environmental issues. At the United Nations Millennium Summit in 2000 and during the 2002 World Summit on Sustainable Development in Johannesburg, world leaders from rich and poor countries, recognized the vital importance of surface and subterranean fresh water to human development, and committed themselves to a precise, time-bound agenda for addressing the world’s current and future water resource and sanitation needs. This promise was reported onto the Millennium Declaration, among the Millennium Development Goals (MDGs). The MDGs are an eight-point1 road map with measurable targets and clear deadlines for improving the lives of the world’s poorest people. World leaders have agreed to achieve the MDGs by 2015 to:. . Integrate the principles of sustainable development into country policies and programs; reverse loss of environmental resources.. . Reduce by half the proportion of people without sustainable access to safe drinking water.. . Achieve significant improvement in lives of at least 100 million slum dwellers, by 2020.. Since, the issues of sustainability and maintenance of quality of drinking water supplied is an area of concern for countries where groundwater is a main source of drinking water, safe drinking water supply has been enlisted as one of the ten targets of Millennium Development Goals (MDGs), to halve by 2015 the proportion of people without sustainable access of safe drinking water. In 2005, for example, slightly more than one third of the urban population in developing regions lived in slum conditions, with the associated problems of inadequate water and sanitation facilities, and lack of social infrastructure, including for health and education. Water use has grown at more than twice the rate of the population for the past century. Although there is not yet a global water shortage, about 2.8 billion people, representing more than 40 per cent of the world’s population, live in river basins with some form of water scarcity (UN, 2008). More than 1.2 billion of them live under conditions of physical water scarcity, which occurs when more than 75 per cent of the river flows are withdrawn. Consequently, especially for rural people, more difficult access to reliable water supplies and high vulnerability to short and long-term drought (figure1.1). 1. The 8 MDGs are: 1- End hunger; 2 - Universal education; 3 - Gender Equity; 4 - Child health; 5 - Maternal health; 6 - Combat HIV/AIDS; 7 - Environmental sustainability; 8 - Global partnership. Daniele Pittalis – Interdisciplinary studies for the knowledge of the groundwater fluoride contamination in the eastern African rift: Meru district, North Tanzania – Natural Science PhD School, University of Sassari. 7.

(8) Figure 1.1 – Proportion (%) of rural households using piped water, other improved sources and unimproved sources, 1990 and 2006 (rearranged from UN, 2008). Fifty per cent of rural dwellers relied on other improved drinking water sources, such as public taps, hand pumps, improved dug wells or springs (a small proportion of this population relied on rainwater). Nearly one quarter (24 per cent) of the rural population obtained their drinking water from ‘unimproved’ sources: surface water such as lakes, rivers, dams or from unprotected dug wells or springs. But even using an improved water source is no guarantee that the water is safe: when tested, the drinking water obtained from many improved sources has not met the microbiological standards set by WHO (UN, 2008). As reported in United Nations 2009, 884 million people worldwide still rely on unimproved water sources for their drinking, cooking, bathing and other domestic activities (figure 1.2). Of these, 84 per cent (746 million people) live in rural areas.. Daniele Pittalis – Interdisciplinary studies for the knowledge of the groundwater fluoride contamination in the eastern African rift: Meru district, North Tanzania – Natural Science PhD School, University of Sassari. 8.

(9) Figure 1.2 - Number of people per year that require access to an improved drinking water source to meet the MDG target, 2006-2015 (rearranged from UN, 2009). Anyway, reducing poverty and achieving sustained development must be done in conjunction with a healthy planet. The Millennium Goals recognize that environmental sustainability is part of global economic and social well-being. Unfortunately exploitation of natural resources such as forests, land, water, and fisheries-often by the powerful fewhave caused alarming changes in our natural world in recent decades, often harming the most vulnerable people in the world who depend on natural resources for their livelihood. Drinking water, for example, continuously is affected by common problems include exposure to toxic inorganic substances, heavy metals, bacterial and other pathogens, increased nitrogen concentrations and other trace chemicals and micronutrients. The chemical. contaminations. are. often. considered. a. low. priority. than. microbial. contamination, because adverse health effects from chemical contaminations are generally. associated. with. long-term. exposure,. whereas. effects. from. microbial. contaminations are usually immediate. The chemicals in water supplies can cause very serious health problems, whether the chemicals are naturally occurring or derived from source of pollution. Groundwater pollution is usually traced back to four main origins: natural (or environmental), agricultural, industrial and residential (or domestic) pollution. Natural: some groundwater pollution occurs naturally even if it is unaffected by human activities. The types and concentrations of natural contaminations depend on the nature of the geological material through which the groundwater moves and the quality of the recharge water. Groundwater moving through sedimentary rocks and soils, for example, may pick up a wide range of compounds such as magnesium, calcium, and chlorides. Some aquifers have high natural concentration of dissolved constituents such as arsenic, boron, and fluoride. The effect of these natural sources of contamination on groundwater quality depends on the type of contaminant and its concentrations.. Daniele Pittalis – Interdisciplinary studies for the knowledge of the groundwater fluoride contamination in the eastern African rift: Meru district, North Tanzania – Natural Science PhD School, University of Sassari. 9.

(10) Agricultural: Pesticides, fertilizers, herbicides and animal waste are agricultural sources of groundwater contamination. Industrial: Manufacturing and service industries have high demands for cooling water, processing water and water for cleaning purposes. Groundwater pollution occurs when used water is returned to the hydrological cycle. Residential: Residential wastewater systems can be a source of many categories of contaminants, including bacteria, viruses, nitrates from human waste, and organic compounds. Similarly, wastes dumped or buried in the ground can contaminate the soil and leach into the groundwater. Natural contamination, due to particular geological environments, can be an important factor in limiting available water resources, both in quantitative and qualitative term, particularly in arid and semiarid areas, where the groundwater is the major source of potable water supply. Its availability may be threatened by natural contaminant as fluorine. In fact, although fluorine can prevent tooth decay, and is often added to drinking water in developed countries, when in excess concentration can lead to fluorosis, a serious health pathologies including malformed bones, neurological disease and may exert some stress on the ecological interrelationships among plant and animal populations in natural biological communities. Sources of fluoride on the Earth’s surface derived not only from natural sources (rock minerals, air, seawater) but also from anthropogenic activities (Fuge and Andrews, 1988).. In a recent study Swiss researchers (Amini et al., 2008) mapped the levels of fluoride and arsenic in groundwater throughout the world (figure 1.3).. Figure 1.3 - Global probability of geogenic flouride contamination in groundwater. More than 20 developed and developing nations are endemic for fluorosis. These are Argentina, U.S.A., Morocco, Algeria, Libya, Egypt, Jordan, Turkey, Iran, Iraq, Kenya, Tanzania, S. Africa, China, Australia, New Zealand, Japan, Thailand, Canada, Saudi Arabia, Persian Gulf, Sri Lanka, Syria, India, etc.. Daniele Pittalis – Interdisciplinary studies for the knowledge of the groundwater fluoride contamination in the eastern African rift: Meru district, North Tanzania – Natural Science PhD School, University of Sassari. 10.

(11) Degradation of groundwater from fluoride, therefore, is one of the most serious water resources problems in Africa. In Tanzania, for example, fluoride in drinking water exceeds both 1.5 mg/L (the limit suggested by WHO guideline) and, in several cases, the 8.0 mg/L (suggested from the Tanzania government). In some of the groundwater supplies of Manyara, Arusha, Mara, Kilimanjaro, Mwanza, Shinyanga, Mbeya and Singida regions, about 90% of the population are affected by dental fluorosis at varying severity or stages. As such, the Tanzania Food and Drugs Authority categorized dental fluorosis as the 5th most common nutritional disorder in the country. Considering that the presence of excessive concentrations of F- in groundwater may persist for years, decades or even centuries (Todd, 1980), in order to mitigate this excess, is essential to determine and monitor the causal factors of enrichment of Fconcentration in groundwater in time and space. This is also one of the main objective of the present thesis’s work : study the spatio-temporal variation of fluoride contents in groundwater of two wards (Ngarenanyuki and Oldonyosambu) of Arusha Region in northern Tanzania and its relationship with some influencing factor, like geological, chemical and physical characteristics of the aquifer and the surrounding environment. Ngarenanyuki and Oldonyosambu areas are involved, from several years, in water distribution and sanitation projects by means of Oikos East Africa (NGO), as limited water resources availability is one of the main problem. In the two wards, the average per capita daily consumption is 8 liters and this value goes down to 3-4 liter per day in the dry period, when most of the population is compelled to concentrate around few water points and cannot resort to temporary ponds or streams. This datum is quite far from the Millennium Goal objectives. Moreover, in these rural areas qualitative water problems occurs, due to the abundance of fluoride concentration, that in many cases exceed the 8.0 mg/L limit. Therefore the results of this study will provide a better understanding about high fluoride concentrations in groundwater, to agree water management plans that aims to: . find new water resources in a area affected by serious water shortage;. . find safe water as in this area the few available water resources are naturally contaminated by high fluoride contents;. . develop a methodology which trough a multidisciplinary approach will satisfy the previous purposes.. In details the work was devoted to assays the possible groundwater resources of the region trough a general hydrogeological model, by means of geological, geophysical and punctual. hydrogeological. data.. Moreover,. trough. the. analysis. of. surface. and. groundwaters, rocks and their weathering products reconstruct the source of fluoride and the factors that control its concentration in the water.. Daniele Pittalis – Interdisciplinary studies for the knowledge of the groundwater fluoride contamination in the eastern African rift: Meru district, North Tanzania – Natural Science PhD School, University of Sassari. 11.

(12) This research was done as part of a project, coordinated by NRD-UNISS (Desertification Research Group- University of Sassari), funded by OIKOS Institute (Italy), Charity and Defence of Nature Fund (private foundation) and Sardinia local Government (Italy) (Regional Law 19/96: cooperation with developing countries).. Daniele Pittalis – Interdisciplinary studies for the knowledge of the groundwater fluoride contamination in the eastern African rift: Meru district, North Tanzania – Natural Science PhD School, University of Sassari. 12.

(13) 2. LITERATURE REVIEW Fluorine is the lightest element of the halogen groups and the most electronegative (Pauling 1960). It is seventeenth in the order of frequency of occurrence of the elements, and represents about 0.06 to 0.09% of the earth's crust (Wedephol, 1974). It is mobile under high-temperature conditions, most reactive of all chemical elements and is therefore, never encountered in nature in the elemental form. As fluoride ions have the same charge and almost the same radius as hydroxide ions, it may replace each other in mineral structures; thus forms mineral complexes with a number of cations and some fairly common mineral species of low solubility contain fluoride (Murray, 1986).. 2.1. FLUORIDE CONTAMINATION The incidence and severity of fluorosis is related to the fluoride content in various components of environment: air, soil and water (figure 2.1).. Figure 2.1 - Rearranged from Edmund and Smedley (2005). In succession the contamination of fluoride in the different environment component is explained.. 2.1.1. Air Air is typically responsible for only a small fraction of total fluoride exposure (USNRC, 1993). Due to dust, industrial production of phosphate fertilizers, coal ash from the burning of coal and volcanic activity, fluorides are widely distributed in the atmosphere.. Daniele Pittalis – Interdisciplinary studies for the knowledge of the groundwater fluoride contamination in the eastern African rift: Meru district, North Tanzania – Natural Science PhD School, University of Sassari. 13.

(14) In non-industrial areas, the fluoride concentration in air is typically quite low (0.05–1.90 μg m–3 fluoride) (Murray, 1986). In China more than 10 million people suffer from fluorosis, related in part to the burning of high fluoride coal (Gu et al., 1990 in Fawell et al. 2006).. 2.1.2. Soil Common source of fluoride in soil proceed from application of phosphate fertilizers, fumigants, rodenticide, insecticides and herbicides containing fluoride as impurity or constituent, e.g., cryolite (used for the production of aluminium), barium fluorosilicate, sodium silicofluoride, sulfuryl fluoride, trifluralin (Datta et al., 1996). Super-phosphate fertilizers may contain F- but this is a minor source, since Rao (1997) found that the contribution from fertiliser was 0.34 mg/L in an area with a maximum of 3.4 mg/L. Very common soil minerals, such as biotite, muscovite, and hornblende may contain as much as several percent of fluoride and, therefore, would seem to be the main source of fluoride in soils (Madhavan, 2001).. 2.1.3. Water sea The sea water has a relatively high fluoride content (1.0 – 1.4 mg/L) as the fluoride is removed by erosion from the continent and transferred to the sea via stream or rivers (Murray, 1986).. 2.1.4. Surface water Generally most groundwater sources have higher fluoride concentrations than surface water, even though, for the later, fluoride levels tend to increase in dry seasons. However closed basins in areas of high evaporation, such as Great Salt Lake, accumulate up to 14 ppm of fluoride, whereas lakes in East Africa formed by leaching of alkali rocks contain 1.000 – 1.600 ppm (Fleischer and Robinson,1963). The primary determinant of surface water fluoride concentration in East Africa, in fact, depends on the weathering processes of fluoride rich rocks (Kilham and Hecky, 1973).. 2.1.5. Groundwater and its hydrogeochemistry Fluoride groundwater is introduced mainly through water–rock interaction in the aquifers (Edmunds and Smedley 1996; Nordstrom et al., 1989; Gizaw 1996; Saxena and Ahmed 2001; Saxena and Ahmed 2003; Carrillo - Rivera et al., 2002), depends on geological, chemical and physical characteristics of the aquifer and the groundwater’s age.. Daniele Pittalis – Interdisciplinary studies for the knowledge of the groundwater fluoride contamination in the eastern African rift: Meru district, North Tanzania – Natural Science PhD School, University of Sassari. 14.

(15) Its concentration is a function of many factors such as availability and solubility of fluoride minerals, velocity of flowing water, temperature, pH, concentration of calcium and bicarbonate ions in water (Chandra et al., 1981), anion exchange capacity of aquifer materials (OH- for F-). The dissolution of minerals, such as fluorspar, fluorapatite, amphiboles (e.g., hornblende, tremolite) and some micas (Datta et al., 1996), offer a considerable contribute to the high content of fluoride. Rocks yielding the highest levels of dissolved fluoride are typically characterized as alkaline igneous rocks, with a high percentage of sodium plagioclase minerals. Such rocks are likely to have formed from magmas enriched in fluorine through progressive differentiation. The predominance of sodium plagioclase is also likely to produce a soft groundwater, which allows higher fluoride concentrations when equilibrium is reached with fluorite. This mineral, controlling aqueous fluoride geochemistry in most environments (Apambire et al., 1997), is one of the major sources of fluoride, although its solubility in fresh water and its dissociation rate are very low (Nordstrom and Jenne 1977). Not uncommonly CaF2 may encounters as a constitution of magmatic rocks (Madhavan, 2001) and often occurs as cement in some sandstones (Rukah and Alsokhny, 2004). The bulk of the element is found in the constituents of silicate rocks, where the complex fluorophosphate apatite, Ca10(PO4)6F2, seems to be one of the major fluoride mineral (Rutherford, et al., 1995). In silicate minerals, as fluorine is concentrated in the last stages of crystallizing magmas, in the residual solutions and vapours, its concentration increase in highly siliceous igneous rocks, alkali rocks and hydrothermal solutions (Fleischer et al., 1963); therefore all these are natural contributors of the fluoride ion to fluids interacting with them, such as groundwater, thermal waters and surface waters. The fluoride in such silicates may even greatly exceed the amount fixed in apatite. Sedimentary horizons also have apatite as accessory minerals (Rukah and Alsokhny, 2004). Next, with regard to fixation of the bulk of fluoride, come some complex hydroxysilicates and hydroxyalumino- silicates, in which the hydroxyl ions (OH) may be largely replaced by fluoride (Omueti, and Jones, 1977), as is the case in amphiboles and minerals of the mica family (biotite and muscovite). Apatite, amphiboles and micas contain fair amounts of fluorine in their structure, which are ubiquitous in igneous and metamorphic rocks. The fluoride content of amphiboles from metamorphic rocks worldwide varies from 30 to 400 ppm (results of various workers, cited by Wedepohl (1978)). Mg-hydroxysilicates unstable minerals, such as sepiolite and palygorskite, also may have a control on F- distribution in groundwater (Wang et al., 1993; Jacks et al., 2005); sepiolite, in particular has been found to contain a considerable amount of F- in the OHpositions. Villiamite (NaF) too, may contribute considerably on groundwater fluoride distribution when associated with certain peralkaline bodies. Other sources of fluoride, are reported. Daniele Pittalis – Interdisciplinary studies for the knowledge of the groundwater fluoride contamination in the eastern African rift: Meru district, North Tanzania – Natural Science PhD School, University of Sassari. 15.

(16) in. literature referred to minerals precipitation both in salt crust like trona, due to. chemical weathering of rock minerals (villiaumite or kogarkoite) and high evaporation of the lake waters (Nielsen, 1999), and calcrete (Jacks et al., 2005). The following table 2.1 account the mean values, in ppm of F, for various geological materials, particularly igneous and sedimentary rocks.. Geological Materials. ppm (average) Igneous. basalts andesites rhyolites phonolites gabbros and diabases granites and granodiorites alkalic rocks. 360 210 480 930 420 810 1.000 Sedimentary. limestones dolomites sandstone and graywackes shales oceanic sediments and soils. 220 260 180 800 730. Table 2.1 - Rearranged from Fleischer and Robinson, 1963. The concentration of fluorine in most basaltic rocks ranges from 0.01 to 0.1 wt% (Allmann and Koritnig, 1974) whereas most granites and rhyolites show a range of 0.01 to 0.2 wt% (Brehler and Fuge, 1974). High fluoride concentrations in groundwater from crystalline basement aquifers, particularly granite, are recognized in several areas of the world: Sri Lanka with up to 10 mg/l, and India, Senegal, Korea, and Wisconsin (US) with up to 7.6 mg/l (Pauwels H. and Ahmed S., 2007). High concentrations of the same halogen, also, can be found whether from groundwater chemical interaction of volcanic rocks and their associates (lahar and ash) or in sedimentary rocks, i.e. in western Senegal (Travi 1993) China, Sudan and Niger, particularly under semi-arid climate conditions. Argentina too, whose groundwater from Quaternary loess of La Pampa recorded up to 29 mg/l of fluoride. Nevertheless, groundwater under more temperate climates of Europe present concentrations above the WHO guideline, but rarely above 4 mg/l: Ledo-Paniselian aquifer in Belgium, Permian carbonates of Lithuania, in the Lutetian limestone and marl aquifer near Paris, as well as in the Bajocian aquifer (Jurassic limestones) in Normandy (western France). Very high concentrations of F(Boyle D.R. and Chagnon M., 1995) was found for groundwater associated with Carboniferous sandstone-siltsone-conglomerate sediments which underlie a thick blanket of alluvial-colluvial-glacial overburden in a area of Quebec (Canada). Most of high fluoride concentration phenomenon in groundwaters, are controlled by pH.. Daniele Pittalis – Interdisciplinary studies for the knowledge of the groundwater fluoride contamination in the eastern African rift: Meru district, North Tanzania – Natural Science PhD School, University of Sassari. 16.

(17) The dissolution of fluoride from apatite, micas, and amphiboles minerals, for example, is most pronounced at low pH values (Apambire et al., 1997); on the contrary, Saxena et al., (2003), performing an experimental study in laboratory about the dissolution of fluoride in granite aquifers of India, showed like alkaline conditions and moderate specific conductivity, were favourable for fluoride dissolution from fluorite to water. Particularly in semi-arid areas, the pH increase influence the groundwater fluoride increment; in Jacks et al., (2005), for example, the raising of pH contributed both the precipitation of calcite and the formation of Mg-rich calcrete and dolomite rich in fluorine (figure 2.2).. Figure 2.2 - Rearranged from Jacks et al. (2005). In Madhavan and Subramanian (2006), was observed that the solubility of fluoride in soils was highly variable and had the tendency to be higher at pH below 5 and above 6. Conversely, Raju et al., (2009) said that in acidic medium, fluoride was adsorbed in clay, whereas in alkaline medium, it was desorbed. In Jacks et al. (2005) was showed a correlation between soil pH and solubility of F- like probable effect of the content of fluoride in the parent material. Soils having high pH and low levels of amorphous Al species, clay, and organic matter generally sorbs little fluoride (Omueti and Jones, 1977). Thus, it appears that the predominant retention mechanism is that of fluoride exchange with the OH group of amorphous materials, such as Alhydroxides (Flühler et al., 1982; Barrow and Ellis, 1986; Bond et al., 1995 and Anderson et al., 1991). The pH factor is, therefore, closely relate to ion exchange. In Boyle and Chagnon (1995) the increasing of pH with decreasing of Ca and Mg concentrations, due probably to the involvement of H+ in the exchange process, turned the water into a strong anion exchange medium for the exchange of OH- for F-, favouring the occurrence of high fluoride concentration. The concentration of Ca, Na, hydroxyl ion and certain complexing ions, in fact, can alter the concentration of fluoride in the groundwaters (Raju. Daniele Pittalis – Interdisciplinary studies for the knowledge of the groundwater fluoride contamination in the eastern African rift: Meru district, North Tanzania – Natural Science PhD School, University of Sassari. 17.

(18) et al., 2009). Any processes involving a decrease in calcium concentration, then, favour the occurrence of high fluoride concentration. This decrease can occurs through ionexchange (substitution of Na by Ca on the mineral surface) during the circulation of groundwater within the aquifer, or through calcite (calcium carbonate) precipitation. Anion exchange is dominant in sedimentary environment but, also, can occur in igneous terrenes (Apambire et al., 1997). Good anion exchange media, from which large amount of fluoride can be generated, are clay minerals like illite, chlorite and smectites (Boyle 1992; Boyle and Chagnon 1995). Usually, high-fluoride groundwater is typically of the sodium-bicarbonate type with relatively low calcium concentrations (< 20 mg/L) and with neutral to alkaline pH values (around 7-9). High concentrations of Na, therefore will increase the solubility of fluorite in waters; in fact (Apambire et al., 1997) sodium, may exhibit a positive correlation with fluoride in many types of groundwater, especially those having low concentrations of calcium (waters undergoing base exchange). Teotia et al., (1981) have reported that water with low hardness, i.e. low Ca and Mg contents, and high alkalinity present the highest risk of fluorosis. Compositional characteristics of the Rift Valley waters, for example, include high alkalinity (pH generally greater than 7) and richness in the components Na, K, HCO3, CO3 as well as Cl- and F- (Gaciri and Davies, 1993). Again, groundwater interactions with fluoride enriched minerals and residence time have been shown important for controlling the fluoride dissociation process. It is generally accepted that fluoride is enriched in groundwaters by prolonged water–rock interactions (Banks et al., 1995; Gizaw, 1996; Nordstrom et al., 1989; Frengstad et al., 2001; Carrillo-Rivera et al., 2002). The chemical composition of lithology, therefore, is regarded as an important factor determining the fluoride concentration of groundwater. Residence time too, can have an important influence on dissolved fluoride levels (Kim and Jeong, 2005; Saxena and Ahmed, 2003; Conrad et al., 1999; Bardsen et al., 1996), because the dissolution rates of fluoride minerals are generally slow (Gaus et al., 2002).. 2.2. FLUORIDE HEALTH PROBLEMS The fluoride absorbed by the human body circulate in the body and then is retained in the tissues, predominantly the skeleton, or excreted, mainly in the urine. Both uptake in calcified tissues and urinary excretion appear to be rapid processes (Charkes et al., 1978).. Low doses (<1.5 mg/l) of fluoride prevent decay of teeth, whereas. concentrations above 1.5 mg/l in drinking water (the maximum tolerance limit of fluoride prescribed by World Health Organization WHO 1984) cause fluorosis and other related diseases (table 2.2).. Daniele Pittalis – Interdisciplinary studies for the knowledge of the groundwater fluoride contamination in the eastern African rift: Meru district, North Tanzania – Natural Science PhD School, University of Sassari. 18.

(19) F Concentration (mg/l). Corresponding effects on human health. ≤1. Safe limit. 1-3. Dental fluorosis. 3-4. Stiff and brittle bones/joints. ≥4. Deformities in knees; crippling fluorosis; bones finally paralysed resulting in inability to walk or stand straight. Table 2.2 – Level of F- content in groundwater and corresponding effect in human health (rearranged from Chaturvedi et al., 1990). Fluorosis is manifested mainly in three ways: fluorosis in soft tissues, such as muscles and ligaments (Kharb and Susheela, 1994), dental fluorosis and skeletal fluorosis. As. reported. by. Meenakshi. and. Maheshwari. (2006),. fluorine. being. a. highly. electronegative element has extraordinary tendency to get attracted by positively charged ions like calcium. Hence, the effect of fluoride on mineralized tissues like bone and teeth leading to developmental alternations, is of clinical significance as they have highest amount of calcium and thus attract the maximum amount of fluoride that gets deposited as calcium–fluorapatite crystals. Tooth enamel is composed principally of crystalline hydroxyhapatite. Under normal conditions, when fluoride is present in water supply, most of the ingested fluoride ions get incorporated into the apatite crystal lattice of calciferous tissue enamel during its formation. The hydroxylion gets substituted by fluoride ion since fluorapatite is more stable than hydroxylapatite. Thus, a large amount of fluoride gets bound in these tissues and only a small amount is excreted through sweat, urine and stool. The length of exposure, frequency of ingestion and ingested fluoride dose determine the plasma fluoride steady state, which in turn influences the severity of dental and skeletal fluorosis of an individual. Skeletal fluorosis may occur when fluoride concentrations in drinking water exceed 4-8 mg/L, which leads to increase in bone density, calcification of ligaments, rheumatic or arthritic pain in joints and muscles along with stiffness and rigidity of the joints, bending of the vertebral column, etc. (Teotia and Teotia, 1988). Grynpas (2008) hypothesize that an increase in bone fluoride affects the mineral-organic interfacial bonding and/or bone matrix proteins, interfering with bone crystal growth and causing inhibition on the crystallite faces as well as bonding between the mineral and the collagen. Attention is also given to the interaction of fluorine with other elements, especially certain metals. Isaacson (2008) reports how fluorides can disrupt semi-independent systems of human nervous system. Of special interest are the anatomical changes induced by fluorides in the brain that resemble alterations found in the brain of Alzheimer’s patients. The. Daniele Pittalis – Interdisciplinary studies for the knowledge of the groundwater fluoride contamination in the eastern African rift: Meru district, North Tanzania – Natural Science PhD School, University of Sassari. 19.

(20) hypothesis is offered that the main cause of all dementias is a reduction in the metabolic activity of the entire brain caused by alterations in blood flow and reductions in chemicals essential to aerobic metabolism. Although water is the epidemiologically most important source of fluoride in most areas, considerable exposure risk is also associated with the consumption of fish-bone, canned meat, vegetables, grain and other staples, local salt, drinks (especially tea) and air. The sources of fluoride. that contribute to the total human intake vary geographically. between endemic fluorosis areas, but the symptoms are generally similar. In nonendemic areas, skeletal fluorosis has occurred as a result of industrial exposure. This condition, whether of endemic or industrial origin, is normally reversible by reducing fluoride intake. In endemic fluorosis areas, developing teeth exhibit changes ranging from superficial enamel mottling to severe hypoplasia of the enamel and dentine (Gittins, 1985). Natural contamination of groundwater by fluoride causes, also, irreparable damage to plant and human health. Fluoride is not an essential plant element, but is essential for animals. Uptake of fluoride in plants mainly occurs through the roots from the soil, and through the leaves from the air. High fluoride levels inhibit germination, cause ultrastructrual. malformations,. reduce. photosynthetic. capacities,. alter. membrane. permeability, reduce productivity and biomass and inflict other physiological and biochemical disorders in plants. Considerable differences exist in plant sensitivity to atmospheric fluoride, but little or no injury will occur when the most sensitive species are exposed to about 0.2 µg/m3 air, and many species tolerate concentrations many times higher than this. Moreover, the continuous use of water having high fluoride concentration also adversely affects the crop growth. For irrigation purpose fluoride is classified according to criteria given by Leone et al., (1948) who proposed a 10 mg L-1 limit for all type of plants Plants, also, are a source of dietary fluoride for animals and human beings.. Thus,. elevation of plant fluoride many lead to a significant increase in animal exposure. Chronic toxicity has been studied in livestock, which usually develop skeletal and dental fluorosis. Experimentally-induced chronic toxicity in rodents is also associated with nephrotoxicity. Symptoms of acute toxicity are generally non-specific. Fluoride does not appear to induce direct mutagenic effects, but at high concentrations it may alter the response to mutagens. Continuous ingestion by animals of excessive amounts of fluoride can lead to the disorder fluorosis, and suboptimal levels in the diet can have an equally damaging effect. The effects of fluoride in drinking water on animals are analogous to those on human beings.. Daniele Pittalis – Interdisciplinary studies for the knowledge of the groundwater fluoride contamination in the eastern African rift: Meru district, North Tanzania – Natural Science PhD School, University of Sassari. 20.

(21) 2.3.FLUORIDE IN THE RIFT VALLEY AND IN TANZANIA Some of the highest fluoride contents in groundwater ever recorded in the world are in the East African Rift Valley with concentrations of up to 20 mg/L in Ethiopia (Wonji/Shoa area), and even more than 100 mg/L in Tanzania (Nanyaro et al., 1984); east Africa (Gaciri and Davies, 1993; Gizaw, 1996), have the most extensive areas of high Fgroundwaters. The high national standard for drinking water in Tanzania reflect the difficulties with compliance a situation that is worsened by water scarcity. In this country both dental and skeletal fluorosis are recognized health problems. Fluoride problems are largely found in groundwater from active volcanic zones, where fluorine sources are imputed at the volcanic rocks and geothermal sources (Edmunds and Smedley 2005). Unmodified waters in the Rift Valley were defined by Clarke et al., (1990) as waters whose chemical composition is derived from normal water-rock interaction at moderate temperatures. These waters showed high fluoride contents (up to 180 ppm), denoting that chemical leaching (weathering) of the volcanic rocks and their associates (calcareous tufa, lahar and ash) was an important fluoride contributor. The volcanic rocks of the Rift System are predominantly alkaline rocks rich in Na+ and F-. Alkali basalts, basanites and tephrites are the main varieties, followed in abundance by phonolites and trachytes (Gaciri and Davies, 1993). Consequently water bodies can accumulate fluoride directly as a result of a weathering of these rocks, as well as from high fluoride geothermal solutions. High fluoride content of waters in Northern Tanzania was attributed (Nanyaro et al., 1984) to the exceptionally low Ca2+ and Mg2+ concentrations due to the low solubility of Ca2+ and Mg2+ fluorides. The Na+-HCO3- rich groundwaters too, derived from weathering of the silicate minerals in the lavas and ashes (Jones et al., 1977) by silicate hydrolysis reactions, are relatively depleted in Ca2+ and Mg2+. Hence high concentrations of fluoride can occur as the solubility of fluorite (CaF2) is not limiting factor. In fact, only limited incorporation of F- is permitted in the CaCO3 structure, such that there is always a net balance of F- in solution. For the computation of thermodynamic equilibrium in groundwaters which are in contact with both calcite and fluorite solid phases, Handa (1975) used a combined mass law equation relating both the solute species as follows:. CaF2 + HCO3-. CaCO3(s) + H+ + 2Fand. K cal . fluor . a HCO3. a H   a F  . 2. ;. Daniele Pittalis – Interdisciplinary studies for the knowledge of the groundwater fluoride contamination in the eastern African rift: Meru district, North Tanzania – Natural Science PhD School, University of Sassari. 21.

(22) If the pH of the groundwater remains reasonably constant, any increase or decrease in bicarbonate concentration/activity will be accompanied by a corresponding increase or decrease in the concentration/activity of fluoride ions, as kcal.-fluor. is constant. Villiaumite (NaF) should limit the dissolved concentrations, but because this mineral is very soluble, fluoride can rich very high concentration before this limit is achieved. High fluoride content for surface water of Tanzania (12-76 mg/L), particularly in the north of the country, was attributed (Kilham and Hecky, 1973) in rivers draining the volcano’s slopes. This high concentrations was due to weathering of fluorine-rich alkaline igneous rocks and to contributions from fumaroles and gases as well as to the redissolution of fluorine-rich trona (magadi), which occurred as a seasonal encrustation in low-lying river valleys and lake margins as a result of extreme evaporation. More recent study, still report the importance of efflorescent crusts magadi and scooped magadi (formed by capillary evaporation) in the water fluoride enrichment (Nielsen, 1999; Kaseva, 2006; Vuhahulaa et al., 2008).. Daniele Pittalis – Interdisciplinary studies for the knowledge of the groundwater fluoride contamination in the eastern African rift: Meru district, North Tanzania – Natural Science PhD School, University of Sassari. 22.

(23) 3. DESCRIPTION OF THE STUDY AREA 3.1. LOCATION AND EXTENT The project involved one portion of the District of Arumeru, that belongs to the Region of Arusha, with an area of approximately 2966 km2. The District is administratively divided into 6 Divisions, 37 Ward and 133 Villages. The district of Arumeru is situated in northern Tanzania, between the Mount Kilimanjaro on the east, the Mount Meru on the south, the road that joins Arusha (Tanzania) with Nairobi (Kenya) on the west and the National Park of Amboseli (Kenya) on the north (figure 3.1).. Figure 3.1 – Location of study area. In particular, the working area (approximately 370 km2) is located in the northern part of the Arumeru district, approximately 50 km from the city of Arusha, is bounded by the Mount Meru (4565 m a.s.l.) and the Arusha National Park, and includes 9 villages belonging to the Oldonyo Sambu and Ngarenanyuki Wards. This area is one of the most important and interesting fields of the Maasai Steppe, a territory extending for more than 200.000 Km2 within the Great Rift Valley (figure 3.2), from the Turkana lake, in Kenya, to central Tanzania, which is traditionally inhabited by Maasai nomad shepherds. The natural environment which characterises the Maasai steppe is mainly savannah with wide plains, hills and volcanic mountain crests. Three main ethnic groups are present: the Wameru, which are farmers, Waarusha and Maasai, which are cattlemen.. Daniele Pittalis – Interdisciplinary studies for the knowledge of the groundwater fluoride contamination in the eastern African rift: Meru district, North Tanzania – Natural Science PhD School, University of Sassari. 23.

(24) Figure 3.2 – The Rift valley. 3.1.1. The Rift Valley A rift can be thought of as a fracture in the earth’s surface that widens over time, or more technically, as an elongate basin bounded by opposed steeply dipping normal faults (Wood and Guth, 2009). Geologists are still debating exactly how the East African rift system (EARS), comes about (Chorowicz 2005). In the EARS the earth’s tectonic forces are presently trying to create new plates by splitting apart old ones. The Nubian Plate makes up most of Africa, while the smaller plate that is pulling away has been named the Somalian Plate (figure 3.3).. Daniele Pittalis – Interdisciplinary studies for the knowledge of the groundwater fluoride contamination in the eastern African rift: Meru district, North Tanzania – Natural Science PhD School, University of Sassari. 24.

(25) Figure 3.3 – The Nubian and Somalian plate (Wood and Guth, 2009). The oldest and best defined rift occurs in the Afar region of Ethiopia and this rift is usually referred to as the Ethiopian Rift. Further to the South a series of rifts occur which include a Western branch, the “Lake Albert Rift” or “Albertine Rift” which contains the East African Great Lakes, and an Eastern branch that roughly bisects Kenya north-tosouth on a line slightly west of Nairobi (figure 3.4). Another south-eastern branch is in the Mozambique Channel. The eastern branch runs over a distance of 2200 km, from the Afar triangle in the north, through the main Ethiopian rift, the Omo-Turkana lows, the Kenyan (Gregory) rifts, and ends in the basins of the North-Tanzanian divergence in the south. The western branch runs over a distance of 2100 km from Lake Albert (Mobutu) in the north, to Lake Malawi (Nyasa) in the south. It comprises several segments: the northern segment includes Lake Albert (Mobutu), Lake Edward (Idi Amin) and Lake Kivu basins, turning progressively in trend from NNE to N–S; the central segment trends NW–SE and includes the basins of lakes Tanganyika and Rukwa; the southern segment mainly corresponds to Lake Malawi (Nyasa) and small basins more to the south. The south-eastern branch comprises N-striking undersea basins located west of the Davie ridge. Most of the great lakes of Eastern Africa are located in the rift valleys, except notably Lake Victoria whose waters are maintained in a relative low area between the high mountains belonging to the eastern and western branches.. Daniele Pittalis – Interdisciplinary studies for the knowledge of the groundwater fluoride contamination in the eastern African rift: Meru district, North Tanzania – Natural Science PhD School, University of Sassari. 25.

(26) Figure 3.4 – The Rift’s series (Wood and Guth, 2009). The two branches together have been termed the East African Rift (EAR), while parts of the Eastern branch have been variously termed the Kenya Rift or the Gregory Rift (Wood and Guth, 2009). The two EAR branches are often grouped with the Ethiopian Rift to form the East Africa Rift System (EARS). It is generally admitted that rift evolution is related to extension. Chorowicz (2005) reports that at more local scale, Bhattacharji and Koide (1987) suggested from theoretical and experimental studies, the development of compressive stress adjacent to and around the active rift zones, due to mantle upwelling and penetrative magmatism. In terms of plate tectonics, block movements in East Africa are divergent, and tension might be considered the major factor (McClusky et al., 2003). Deformation of the continental lithosphere, leading to rupture, has been theoretically conceived to occur by two possible ways (Burke and Dewey, 1973; McKenzie, 1978), considering the role played by the asthenosphere. (1) Active rupture would result from mantle convection (Pavoni, 1993) and plume movements in a dynamic asthenosphere that forcibly intrudes and deforms the overlying lithosphere. (2) Passive rupture model sees the asthenosphere uplift playing an entirely responsive role in filling the gap produced by lithospheric extension, itself a reaction to stresses generated elsewhere at plate boundaries due to external forces. Models of tectonic rupture range from whole lithosphere simple shear (e.g., Wernicke and Burchfield, 1982;. Daniele Pittalis – Interdisciplinary studies for the knowledge of the groundwater fluoride contamination in the eastern African rift: Meru district, North Tanzania – Natural Science PhD School, University of Sassari. 26.

(27) Lister et al., 1986) to combination of upper brittle layer simple shear, lower crustal delamination and lithospheric mantle pure shear (e.g., Lister et al., 1986). Wood and Guth (2009) also assumes that elevated heat flow from the asthenosphere is causing a pair of thermal “bulges” in central Kenya and the Afar region of north-central Ethiopia. As these bulges form, they stretch and fracture the outer brittle crust into a series of normal faults forming the classic horst and graben structure of rift valleys. These successions of graben basins are generally bordered on the two sides by high relief, comprising almost continuous parallel mountain lines and plateaus, and sometimes volcanic massifs. As shown in figure 3.5, the elevated areas belonging to the EARS are comprised in two ellipses, one is the Ethiopian dome, and the other includes the Kenyan and Tanzanian domes. The longest axes of the two ellipses (Chorowicz, 2005) have a NNE trend: at this scale the main expression of the EARS is uplift, forming on the whole a NNE-trending intra-continental ridge, interrupted by the Omo-Turkana lows. The highest elevations in the EARS region, in addition to volcanoes, are the graben shoulders. Other uplifted areas in the region are due to belts, which may be recent (Atlas, Zagros belts) or ancient (Karroo belt), or to intracontinental hotspots (Hoggar, Tibesti plateaus).. Figure 3.5 – Map of Africa showing in blue levels the elevations higher than 1200 m, evidencing the main Ethiopian and Kenyan–Tanzanian domes (from Chorowicz, 2005). Ideally the dominant fractures created occur in a pattern consisting of three fractures or fracture zones radiating from a point with an angular separation of 120 degrees (the triple junction) and is well illustrated in the Afar region of Ethiopia (figure 3.6), where. Daniele Pittalis – Interdisciplinary studies for the knowledge of the groundwater fluoride contamination in the eastern African rift: Meru district, North Tanzania – Natural Science PhD School, University of Sassari. 27.

(28) two branches are occupied by the Red Sea and Gulf of Aden, and the third rift branch runs to the south through Ethiopia. The rifting of East Africa, as mentioned, involves the two branches: one to the west, which hosts the African Great Lakes (where the rift filled with water) and another nearly parallel rift about 600 kilometers to the east which nearly bisects Kenya north-to-south before entering Tanzania where it seems to die out (figure 4). Lake Victoria sits between these two branches. It is thought that these rifts are generally following old sutures between ancient continental masses that collided billions of years ago to form the African craton and that the split around the Lake Victoria region occurred due to the presence of a small core of ancient metamorphic rock, the Tanzania craton, that was too hard for the rift to tear through. Because the rift could not go straight through this area, it instead diverged around it leading to the two branches that can be seen today. The Neogene tectonics and volcanism in the rift area of northern Tanzania are intimately related. A major phase of late Tertiary faulting, giving rise to a broad tectonic depression, was followed by extrusion of large amounts of basaltic to trachyte magmas from large shield volcanoes. This was separated by a second major phase of faulting at about 1.2 Ma from a Late Pleistocene-Recent phase of small volume, explosive nephelinitephonolite-carbonatite volcanism that contrasts with the earlier phase in its volume, dominant magma type and eruptive style. In both its tectonic expression and contemporaneous magmatism, the northern Tanzania province contrasts with the southern Kenya sector of the Rift Valley. The area of tectonic disturbance is considerably broader in Tanzania where ultrabasic-basic magmatism predominates. The major episodes of basaltic magmatism representing major thermal perturbations of the mantle, have moved southwards down the rift since the mid Tertiary (Dawson, 1992).. Daniele Pittalis – Interdisciplinary studies for the knowledge of the groundwater fluoride contamination in the eastern African rift: Meru district, North Tanzania – Natural Science PhD School, University of Sassari. 28.

(29) Figure 3.6 – The “triple junction” (Wood and Guth, 2009). In northern Tanzania, the rift widens from about 50 km, as is typical in southern Kenya, to approximately 200 km wide splitting into three distinctively oriented branches: the Natron–Manyara– Balangida, the Eyasi–Wembere and the Pangani rifts. The change in the rift morphology is thought to be a result of the transition from the rifting of Proterozoic Mozambique Belt lithosphere to the rifting of cratonic Archean lithosphere (Foster et al., 1997). Faulting along the East African Rift is considered to have commenced in the Miocene, approximately 13 Ma (Davidson and Rex, 1980; Courtillot et al., 1984). Present day faulting along the Gregory Rift in northern Tanzania is thought to have begun by about 1.2 Ma, and is superimposed on an earlier episode of faulting that began at about 3 Ma (Dawson, 1992; Foster et al., 1997). The earliest evidence of volcanic activity along the Gregory Rift in northeastern Tanzania is associated with the Miocene phase of rifting, with the eruption of 8.1 Ma phonolitic lavas at the centrally located Essimingor volcano (Bagdasaryan et al., 1973). Volcanic activity appears more widespread in the Plio-Pleistocene, with the eruption of alkali basalt–trachyte-phonolite association lavas. In the vicinity of the Ngorongoro Volcanic Highland situated at the southern end of the Gregory Rift, volcanic activity occurred at Satiman, Lemagurut, Ngorongoro, Olmoti, Embagai, Loolmalasin, Oldeani, and Oldonyo Sambu, Terosero, Kitumbeine, Gelai, Meru and Kilimanjaro (Godwin et al., 2008). Daniele Pittalis – Interdisciplinary studies for the knowledge of the groundwater fluoride contamination in the eastern African rift: Meru district, North Tanzania – Natural Science PhD School, University of Sassari. 29.

(30) Godwin et al., (2008) also describe the third phase of volcanic activity occurred after the 1.2 Ma faulting event. Unlike the earlier phase of eruptions, this was smaller in volume and highly explosive. Pyroclastic volcanic cones of Meru, Monduli, Oldonyo Lengai and Kerimasi are thought to be part of this phase of volcanic activity. The magma is ultrabasic to ultra-alkaline in composition and the rocks are mainly phonolites and feldspathoidal syenites.. 3.2. HYDROMETEOROLOGY Average annual precipitation is about 1000 mm, although 50% of the country receives less than 750 mm; in general, rainfall decreases from north to south. Climate is generally semi-arid, with two different seasons: the dry season and the rain season, with rainfall ranging from 400 mm/year in Makami to 1500 mm/year in Ngorongoro. Rains are concentrated between November and December (the so-called small rains or mvuli in Swayli language) and March-May (big rains or masika). January, Septemebr and October are, normally, the hottest months of the year. Annual temperature ranges from 20 to 28 °C. Particularly, the study area despite its proximity to the equator, enjoys an Afro-Alpine temperate climate, characterized by two distinct seasonal weather patterns. The main wet season extends from February to mid May and contributes to about 70% of the total annual rainfall. A minor rainy season from September to November contributes the rest of the moisture in the region. The remaining months of the year are more or less dry, at times with occasional, erratic showers. The lowest and the highest annual average temperatures, are 20.6 °C and 28.5°C, respectively, and the mean yearly rainfall is around 400 mm (figure 3.7), as inferred by 30 years of systematic rainfall measurements achieved from WorldClim database, a set of global climate layers (climate grids) with a spatial resolution of a square kilometer (Hijmans et al., 2005).. Daniele Pittalis – Interdisciplinary studies for the knowledge of the groundwater fluoride contamination in the eastern African rift: Meru district, North Tanzania – Natural Science PhD School, University of Sassari. 30.

(31) Figure 3.7 - Mean yearly rainfall. 3.3. GEOMORPHOLOGY The topography of the area is dominated by the volcanic cone of the mount Meru (4565 m a.s.l.); its slopes, cover most of the area.. Mount Meru is a young volcano of. Pleistocene to recent origin, located on a traverse branch of the southern Gregory Rift. The genesis of the mountain can be distingued by a number of a different development stages. The first was as explosive, creating a yellow tuffs. This was followed by an alternation of multiple eruptions that formed the main body of the mountain to a height considerably above the present, with snow and ice on the top. The next stage included a collapse of the summit and upper E side of the mountain. The rock masses lubricated with water, flowed out over the plains between Mt. Meru and Mt. Kilimanjaro, creating the present lahar topography in that area. This area is characterized by a complex of mounds and ridges, the depression between them often being occupied by lakes or swamps. A paleolimnological study of some of these alkaline lakes dates the bottom sediments to only 6000 years, indicating this recent age of the lahar formation and collapse of the mountain (Hecky, 1971). After the collapse a number of lava outflows have occurred, the latest dates back to 1910, when small amounts of black ash were ejected for a few days from the Ash Cone. Likely most of the recent lava activity took place during the half-century prior to the ash eruption in form of lava domes. Up to 1954 significant fumarolic activity was recorded in the Ash Cone area. In 1974 a careful survey showed no fumarolic activity and anomalous soil temperature.. Daniele Pittalis – Interdisciplinary studies for the knowledge of the groundwater fluoride contamination in the eastern African rift: Meru district, North Tanzania – Natural Science PhD School, University of Sassari. 31.

(32) The remaining land is overlain by the alluvial fans, which, with gentle slope, are fed by Mount Meru detritus. Recent small volcanic cones are preserved in the NW part of Meru and small maar-type flat craters occur (i.e. in front of Mt. Songe).. Permanent saline. water characterise the Big Momela lake and the Small Momela lake, both inside the Arusha National Park, east of Meru.. The drainage pattern around the Meru is clearly. radial, but downhill the stream courses are modified by tilting and capture.. East and. north-east of the Mt. Meru, the only perennial is the Engare Nanyuki river, which flows northwards into the inner Amboseli Basin.. 3.4. GEOLOGY The age of the rock sequences of the study area is Cenozoic: in particular, the older ones go back to Miocene-Pliocene; whereas, the most recent are sub-actual. The dominant lithology is represented by volcanic rocks and, subordinately, by recent alluvial deposits. The crystalline basement does not crop in the area; however it has been found at shallow depth, few kilometers north of the study area.. 3.4.1. Litostratigraphy The lithologies of the area (figure 3.8) can be summarized as follows: The Basement No crystalline Basement rocks are exposed in this area. However, a borehole log west of the Engare Nanyuki river shows pegmatite and gneiss at a depth of about 15m. This evidence, together with the extensive outcropping of Basement only few kilometres to the north, implies that metamorphic rocks underlie the northen part of the study area, very likely at small depths.. In addition, at several localities (Matuffa Crater, Olijoro. Crater and Meru Caldera), lavas and pyroclastic rocks enclose small pebbles to large blocks of Basement gneiss. Volcanic rocks Older and Younger extrusives rocks have been distinguished, based on the relationships with the main phase of rifting and faulting, that should date back to 1.15-1.20 million years. However, the few radiometric ages and field evidences that support this correlation, are still provisional.. Daniele Pittalis – Interdisciplinary studies for the knowledge of the groundwater fluoride contamination in the eastern African rift: Meru district, North Tanzania – Natural Science PhD School, University of Sassari. 32.

(33) Older Extrusives The Older extrusives include the faulted plateau region of the Flood lava group (Nvz) and the Meru West group (Nvm). The first lavas are quite well exposed in fault-scarps, far from the study area. Lavas belonging to the Meru West group appear as a block-faulted structure emerging from below the Meru. Thick ash fall deposit covers the top and the sides of this volcanic complex, that is exposed only in a few steep scarps where thick sub-horizontal nephelinite lavas and breccias crop. These lavas date back to 1,5 million years BP, and then should be younger than Flood lavas pertaining to Nvz. The associated breccias, however, contain dominant phonolite clasts which have been dated back to 2,0 million years BP, indicating a hidden series of alkaline lavas which must have erupted in concomitance of, or just after, faulting (2,1 million years). The crater-like summit plateau of Meru West is of uncertain origin. Younger Extrusives The earliest volcanic activity after the main rift faulting is represented by the phonolites and phonolitic nephelinites of Oldonyo Sambu (Nv); they also crop north of Naigonesoit. Later formations hide the lateral continuity of NV, but it is argued that the formation stretchs further eastwards, since phonolitic nephelinite clasts, in the breccias of Little Meru (Nvp), are coeval (300.000 years BP). Little Meru is a monogenetic volcanic cone rising to more than 3795m a.s.l. from the NE flanks of Meru.. The slopes are very. symmetrical and, although the basal relations cannot be seen, it probably was completely built and extinct before being partly buried by the later Meru lavas. The rock is a very uniform breccia with clasts of phonolitic nephelinite. The Meru centre is located to the south where, between about 200,000 and 80,000 years BP, the built up of the actual main cone took place, namely a large and fairly symmetrical cone to an altitude of at least 4877 m a.s.l., perhaps considerably higher in the past. The Main Cone group (Nvm) materials are predominantly volcanic breccias and tuffs of all size-grades, but phonolitic and nephelinitic lavas are intercalated sporadically. The loose nature of much of the original pyroclastic material, resulted in a radial redistribution outwards into fan and fluvio-volcanic sediments. Lahars (Nzd1) of considerable extension commonly generated, interbedded with alluvial sequences, often over large areas. Extesive lahars are those of Temi-Burka valley, Tengeru and Engosomit and Lemurge. The latter are characterised by large and abundant boulders and by feldsparphyric phonolite with alkali feldspar phenocrysts up to 5 cm in diameter. derives from a concealed portion of the Button Hill tholoid.. This rock probably. These lahars are not of a. single origin, some being volcanic, some sedimentary. Another feature of the Main Cone group is the common occurrence of viscous domes or tholoids (Nvg), usually of a feldsparphyric phonolite composition. These may occur at all levels, but there is zone of especially large adventitious tholoids on the northern flanks of Meru.. Daniele Pittalis – Interdisciplinary studies for the knowledge of the groundwater fluoride contamination in the eastern African rift: Meru district, North Tanzania – Natural Science PhD School, University of Sassari. 33.

(34) The completion of the main cone was followed by a period of deep gullying and erosion with a recrudescence of activity about 60,000 years BP.. The Summit group (Nvn),. predominantly of thick phonolitic and nephelinitic lavas, formed a capping of the summit of the main cone, now largely gone, with a few more massive flows persisting further down the flanks, where they now form prominent ridges. At some undetermined time, subsequent to the Summit group activity, the whole upper part of the Meru cone collapsed, giving rise to huge lahars east of the volcano. These deposits cover about 1500 km2; they travelled about 50 km to the north and 30 km to the south of the cone and washed up against the lower slopes of Kilimanjaro to the east. The collapse did not occur as a single event and the last phase produced the Momela lahar (Nzd3), which flowed farther east out of the graben and gives rise to the characteristic mounded topography of the National Park. The episode has been dated at about 7000 years BP by radiocarbon dating of bottom sediments from one of the Momella lakes.. It is thought that the mantling ash (Nvf) derives from a plinian eruption. associated with the primary collapse and, at its base, there is a concentration of pumice lapilli representing juvenile magma.. The ash is thickly deposited over much of the. mountain, but especially to the west, even beyond the western margin of the area. In the study area, the tuff is notable for its bright yellow colour, but elsewhere the colour grades to a brown, making difficult a precise correlation. This ash has the unfortunate effect of obscuring much critical geology. There are numerous examples of forms which are manifestly buried volcanic cones, some even retaining the morphology of craters, which display no exposures of underlying structure. The final phase of activity was prevalently restricted to the collapse caldera. Mainly cinder and ash activity built up the Ash Cone to about 1067 m above the Caldera floor. At a late stage, a lava dome formed between the Ash Cone and the Caldera wall, from which nephelinitic and phonolitic lavas have flowed over the caldera floor and down to the graben.. A flank eruption, apparently of the same magma originating SE of Little. Meru, flows for some considerable distance. Other volcanic centres Parasitic cone are a notable feature of the region; there are a considerable number of cones with ankaramitic or picrite-basaltic affinity. In particular, a group of cones is of phonolitic affinity and is found on the lower flanks of Meru, with which the activity is closely related. The same activity is encountered north of Meru where a block-faulted plateau of laharic debris has been pierced, over a small area, by shallow volcanic craters. Rims are comparatively small breccia and tuff; these maars are clearly the result of mainly gas action, probably phreatomagmatic in origin.. Daniele Pittalis – Interdisciplinary studies for the knowledge of the groundwater fluoride contamination in the eastern African rift: Meru district, North Tanzania – Natural Science PhD School, University of Sassari. 34.

(35) Superficial deposits Objectively, the interpretation and representation of the geology of this area lies in the lateral gradation of sub-aerial pyroclastic materials of Meru into fluvio-volcanic and lacustrine deposits. Boundaries are often representational and may have been drawn at an appropriate break of slope.. The classification “alluvium” has been used for rather. diverse deposits which reconnaissance mapping cannot attempt to subdivide. There are present some black soils with distinctive carbonate concretions; soils on volcanic rocks show substantial colour variation from red to brown and even grey. Basaltic cones are commonly fringed by a zone of calcrete (calcareous duricrust).. Figure 3.8 – Geological map of the study area. 3.4.2. Geological Structure Among the three distinct domains that can be distinguished in the S Kenya–N Tanzania rifted area, the study area fall down in 200×50 km transverse volcanic belt extending at N80°E from the Ngorongoro crater to the Kilimanjaro; includes numerous (<20) volcanic edifices, and their extensively distributed effusive and air-fall material, that were emplaced during the time interval 8 Ma–Present. The NKVB (Ngorongoro–Kilimanjaro transverse volcanic belt) is very little deformed and shows an inhomogeneous distribution. Daniele Pittalis – Interdisciplinary studies for the knowledge of the groundwater fluoride contamination in the eastern African rift: Meru district, North Tanzania – Natural Science PhD School, University of Sassari. 35.

(36) of extensional faulting. In its eastern part the N80°E trend is exclusively expressed by three aligned volcanoes (Monduli, Meru and Kilimanjaro) forming the Meru lineament. Throughout the NKVB, fault structures are inhomogeneously distributed. Monduli and Meru edifices are transected by minor transverse fault structures. Major rift faults are present in the NW outside part of the area (Matuginigi and Matisiwi Escarpment). Linear features and benches are frequent on the flanks of Meru and it is highly probable that the early volcanic structure has been block-faulted. In the central area, the faulting is N-S to NNE-SSW (Uwiro graben); in the NW area the faulting is NWSE (parasitic cone in Lassarkartarta). However, thick mantling ash and other younger formations make it difficult to map faults with confidence. The date of this faulting must lie between that of the flood lavas (2.3 million years BP) and that of parasitic cones (1.7 million years BP), some of whose lavas cover the fault scarp. This is consistent with the fault-phase found elsewhere about 2.1 million years BP.. 3.5. HYDROGEOLOGY In the study area, the main aquifer systems are made up of volcanic formations, occurring singularly or superimposed each other.. Subordinate perched aquifers are. present in sedimentary formations. However, some of these aquifer systems have a local occurrence. The thickness of the volcanic rocks is known only approximately because of the uncertainties associated with the geologic and geomorphologic events during the Cenozoic. However, it is clear that all these events exerted a strong control on the geometry of the aquifers, on the recharge and discharge areas and on the groundwater quality. Moreover, the scale of the map and the amount of available data does not allow a detailed mapping of all these topics. From a hydrogeological point of view, the litho-stratigraphic formations, described above, can be grouped in two main hydrogeologic unit:. . volcanic hydrogeologic unit;. . sedimentary hydrogeologic unit.. Volcanic hydrogeologic unit This unit is divided into four hydrogeologic complexes: . Meru west Group (Nvm);. . Lahars of various age (Nzd1), Ngare Nanyuki lahars (Nzd2), Momella Lahar (Nzd3);. . Main cone group (Nvm), Ash cone group (Nvn);. . Mantling ash (Nvf);. Daniele Pittalis – Interdisciplinary studies for the knowledge of the groundwater fluoride contamination in the eastern African rift: Meru district, North Tanzania – Natural Science PhD School, University of Sassari. 36.

(37) Meru west Group (Nvm) complex This formation, that belongs to “older extrusive”, is exposed in the west side of Meru. The rock are essentially nephelinite lavas and breccias that contain dominant phonolite clasts. The aquifer hosted in this formation has a fractured permeability. Springs, with a good quality water, are fed by this aquifer. Lahars of various age (Nzd1,Nzd2,Nzd3) complex From a hydrogelogic point of view, all these lahars can be grouped into a unique complex.. Lahars. (Nzd1). of. considerable. extension. were. commonly. generated. interbedded with sedimentary sequences. These lahars are characterised by large and abundant boulders on the surface, of a feldsparphyric phonolite with alkali feldspar phenocrysts and are not of a single origin, some being volcanic, some sedimentary. North-East and East of the volcano, Nzd2 and Nzd3 are exposed. The first, near Ngare Nanyuki and Uwiro graben, the second near the Momella Lakes. The aquifers hosted in this rocks have double permeability (fractured and porous) and generate springs with high fluoride concentration. Also some hydrothermal springs occur within this complex. Main cone group (Nvm) complex The Meru became active in a period ranging about 200,000-80,000 years BP. The volcanic activity built up the main cone to an altitude of at least 4877 m asl, perhaps considerably higher at one time.. The Main Cone group (Nvm) materials are. predominantly volcanic breccias and tuffs of all size-grades, but phonolitic and nephelinitic lavas are intercalated sporadically. The aquifer hosted in this rocks presents double permeability (fractured and porous). Springs, with low fluoride concentration, are present. In this system there are some important hydrogeological evidences. The first one is the elevations difference between recharge and discharge area that allows the infiltration of rain water, particularly where the permeability is high (intensive fracturing). The second one is the number of springs with important yield and good quality (low fluoride concentration and no human activities are present in the recharge area). Another feature of the Main Cone group is the common occurrence of viscous domes or tholoids (Nvg), usually of a feldsparphyric phonolite composition. These may occur at all levels, but there is a zone of especially large adventitious tholoids on the northern flanks (i.e. M. Songe).. The occurrence of domes constitutes a lateral hydrogeological. impermeable limit, that controls the groundwater circulation.. Daniele Pittalis – Interdisciplinary studies for the knowledge of the groundwater fluoride contamination in the eastern African rift: Meru district, North Tanzania – Natural Science PhD School, University of Sassari. 37.